What defines a community?

Via the “They Said It” column in Linux Journal Issue 161 (Sept. 2007 - I’m a bit behind in my reading), I came across this bit of wisdom from Adam Fields on The First Rule of Community:

There’s really only one rule for community as far as I’m concerned, and it’s this - in order to call some gathering of people a “community”, it is a requirement that if you’re a member of the community, and one day you stop showing up, people will come looking for you to see where you went.

Brilliant, though as he himself notes there are many offline communities where the same could not be said. (Come to think of it, I believe I’ve worked in companies which might fail that test).